Peavey TKO 115 repair help

[brothercactus]

Hello

I just picked up a peavey TKo 115 80w bass amp from a guy becasue it's not working. I opened it up and theres a 3am 250volt fuse thats blown. I slapped a new one in and it blew as soon as i switched the amp on.

theres a schematic here

http://music-electronics-forum.com/t14724/

I'm handy, but electronics repair is kinda news to me. How safe is it to use a multimeter around the main power input?

thanks

[Enzo]

Safe? Safe enough, all an amp does is provide voltages. You can measure them with a meter.

Disconnect the speaker until the problem is solved. Work with the amp unplugged from the wall. Check the main power transistors. ANy shorted? I bet they are.

[brothercactus]

gotcha

I'm pretty green....

The power transistors are the 2 the "buttons" mounted on the heat sink right?

How do i test for a short with the meter? I have a simple analog multimeter that i'm getting used to using.

There are 2 screws in the top of the transistor. If i undo the screws will that allow me to remove the transistor? If not it looks like i need to get under the board to test the leads on the transistors...

I'm also digging google for how to test a transitor

[brothercactus]

Well shit...... First of all, i'm going to want to start by making sure the battery is in the multimeter the right way.......


Now, i'm done for the night.
What i got done after i flipped the battery around though:
transistors soldered in...
The board has been carefully freed and turned upside-down to expose the leads. I read many web articles about testing the transistors. Now that the battery is in the right way: There seems to be NO resistence in the transistors. All combinations of of the 3 leads on the transistor show the same resistence, on the meter it's the same as touching the 2 probes.

edit:I was thinking about it. Do i need to remove the transister from the board to test it accurately?

I read a lot about what you'll see whan the transistor works, but what will i see when it's not working?
I'm learning as i got and i just figured out a bunch of stuff. I might still be lost but thats what i've got till later.

That schematic is appreciated by the way

[Enzo]

You measure between the two legs of the transistor and from each leg to the casing (the two screws). While it is in circuit, if any combination appears shorted together - zero or very low ohms resistance - then you remove the transistor and test it on its own.

[brothercactus]

I've removed both the transitors.

The one shows no resistance at all! evey connection seems to be shorted. (this may be my fault maybe, first thing i did when i got the amp was dove in with a multimeter with the power ON! could that have blown the transistor if i shorted it with the multimeter?)

The other looks fine except the collector and the emitter connection should show high resistance either way? it appears to show no resistence either way, between these two leads. I'm still makeing sure but i'm pretty confident thats what i've got.

If I didn't blow the one, Is it ever normal for both transitors to blow or would you suspect I have other problems to find?

They look like there different transistors too.i'm dealing with q5 and q7 on the schematic one says 73100 and other is 83100.

Now i'm trying to find these parts. Think radioshack has'em? I was in there the other day and they DO still have some transistors.

Thanks

[Enzo]

CALL the parts department at Peavey and order the parts there.

[brothercactus]

ya, noway did RadioShack have like ANyThing!

I'll call Peavey on Monday.

[brothercactus]

ordered output transistors. in about 10 days i'll take this thread back up.

for the record, peavey is very helpful on the phone!

radio shack sucks, also my local peavey dealer is a joke.

[RWood]

Did putting in the new output transistors make the amp work?


RWood

[plec.22]

Brothercactus.. it's a common thing to see with most ss state amps that there o/p transistors are shorted and a blown the fuse is a good indication of that and putting new fuse in only to see the big flash as it pop's tell's us there is a big fault...that combo amp has the same type transistor within it's o/p stage npn's so fitting these types will help 2n3237's or mj15024 will do... using your meter check the rest of the transistors.. using a 60/100w bulb limiter between the ac in of the amp and powering with glow bright showing fault and a quick bright glow then to low glow with give time to carry out voltage test's..

This amplifier has two supply lines ht to feed the power stage and lt to feed the pre-amp stage +/- 15volts.. you will see plug in connectors so before pull them out just mark with a pen were they all go or take a picture just in case you get called away and forget where they went...

Enzo is right retest the power transistors out of circuit but a short o/p will show up in circuit just remove the pair and replace them.

[fewa]

Peavey TKO 65 Amplifier

[fewa]

TKO 115